This invention relates to a process for the preparation of 6-(2,3-dichlorophenyl)-1,2,4-triazine-3,5-diamine of the formula I: 
commonly known as Lamotrigine.
Lamotrigine, an anti-epileptic drug, elicits its action by suppressing seizures by inhibiting the release of excitatory neurotransmitters. Lamotrigine presently offers a worthwhile alternative for treating patients suffering from nitractable partial seizures coupled with or without secondary generalised seizures and therefore shows good potential for broader applications in other areas of epilepsy management.
One method of preparation of lamotrigine of the formula I involves reaction of 6-(2,3-dichlorophenyl)-5-chloro-3-thiomethyl 1,2,4-triazine of the formula II: 
with ethanolic ammonia in a sealed tube at 180xc2x0 C./250 psi pressure (PCT Publication No WO 96/20935). This process is time consuming (xcx9c72hours) and also produces lamotrigine in low yields because of which it is not commercially viable.
Another route for the synthesis of lamotrigine of the formula I involves photochemical reaction of the compound of the formula III: 
where R=CN or CONH2, using ultraviolet or visible radiation in the presence of a base in an alkanol solvent and also heating when R=CN (PCT Publication No WO 96/20934). The preparation of the compound of the formula III involves expensive and hazardous reagents. Further, undesired by-products like the de-aminated hydroxy derivative of triazine formed during the photochemical reaction demand elaborate separation and purification techniques, thereby making this route lengthy and tedious, besides producing low yields of lamotrigine ( less than 10%). Therefore this process is not Able for industrial scale manufacture of lamotrigine.
Yet another method for the synthesis of lamotrigine of the formula I involves cyclisation of the Schiff""s base of the formula IV: 
by refluxing in C1-C4 aliphatic alkanol in the presence or absence of a strong base such as KOH (EP Patent No 21121 and U.S. Pat. Nos 4,602,017 and 4,847,249).
The Schiff""s base of the formula IV may be prepared by a sequence of steps comprising:
(1) reaction of 2,3-dichloroiodobenzene of the formula V: 
with magnesium, followed by reaction of the resulting Grignard moeity with solid carbondioxide;
(2) reaction of the resulting 2,3-dichlorobenzoic acid of the formula VI: 
with thionyl chloride in an inert atmosphere such as moisture free nitrogen gas;
(3) reaction of the resulting 2,3-dichlorobenzoyl chloride of the formula VII: 
with a metal cyanide and alkali metal iodide such as Cu(one)CN and KI in the presence of an organic solvent such as xylene in an inert atmosphere such as nitrogen; and
(4) reaction of the resulting 2,3-dichlorobenzoylcyanide of the formula VIII: 
with aminoguanidine bicarbonate in an organic solvent such as DMSO in aqueous acidic medium using 8N HNO3. The purification of crude lamotrigine of the formula I thus obtained by cyclisation of the Schiff""s base of the formula IV is carried out by recrystallisation from isopropanol (FP Patents Nos 59987 and 21121 and U.S. Pat. Nos 4,602,017 and 3,637,688).
The formation of 2,3-dichlorobenzoic acid of the formula VI for the preparation of the Schiff""s base of the formula IV by the above route demands a dry environment thereby making the process laborious. These reactions leading to the Schiff""s base of the formula IV also employ expensive and hazardous reagents like DMSO in large quantities and xylene. The conversion of 2,3-dichlorobenzoyl chloride to 2,3-dichlorobenzoyl cyanide takes 96 hours thereby making the entire process for the synthesis of the Schiff""s base from 2,3-dichlorobenzoyl chloride time consuming (xcx9c7.5-10 days). This route also produces low yields of lamotrigine (xcx9c10%). Therefore this process for the preparation of lamotrigine is not feasible for industrial scale manufacture.
The Schiff""s base of the formula IV may also be prepared by the reaction of 2,3-dichlorobenzoyl cyanide of the formula VIII with aminoguanidine bicarbonate in the presence of acetonitrile and dilute aqueous sulfuric acid (U.S. Pat. No 4,847,249). This route for the synthesis of the Schiff""s base is reported to produce low yields of lamotrigine.
As lamotrigine has emerged to be one of the promising anti-epileptic and anti-convulsant for treating CNS disorders, its commercial production assumes significance. Despite the several routes known for the synthesis of lamotrigine there is still need for a route which is safe, convenient, efficient, economical and less time consuming.
An object of the invention is to provide a process for the preparation of 6-(2,3-dichlorophenyl)-1,2,4-triazine -3,5-diamine of the formula I, commonly known as lamotrigine, which is safe and convenient.
Another object of the invention is to provide a process for the preparation of 6-(2,3-dichlorophenyl)-1,2,4-triazine-3,5-diamine of the formula I, commonly known as lamotrigine, which is less time consuming.
Another object of the invention is to provide a process for the preparation of 6-(2,3-dichlorophenyl)-1,2,4-triazine -3,5-diamine of the formula I commonly known as lamotrigine, which is efficient and economical.
Another object of the invention is to provide a process for the preparation of 6-(2,3-dichlorophenyl)-1,2,4-triazine-3,5-diamine of the formula I, commonly known as lamotrigine, which is suitable for industrial scale manufacture.
According to the invention, there is provided a process for the preparation of 6-(2,3-dichlorophenyl)-1,2,4-triazine -3,5-diamine of the formula I: 
commonly known as lamotrigine which comprises:
a) reduction of 2,3-dichloronitrobenzene of the formula IX: 
in C1-C6 aliphatic alkanol with hydrogen gas at a pressure of 55-90 psi in the presence of a metal catalyst at 27-35xc2x0 C.;
b) diazotisation of the resulting 2,3-dichloroaniline of the formula X: 
with sodium nitrite and a mineral acid at xe2x88x925xc2x0 to 5xc2x0 C. followed by cyano-de-diazonation with a metal cyanide at 65-80xc2x0 C.;
c) hydrolysis of the resulting 2,3-dichlorobenzonitrile of the formula XI: 
under acidic or alkaline conditions;
d) chlorination of the resulting 2,3-dichlorobenzoic acid of the formula VI: 
with a chlorinating agent at 55-130xc2x0 C.;
e) cyano-de-halogenation of the resulting 2,3-dichloro-benzoyl chloride of the formula VII: 
with a metal cyanide in the presence of an alkali metal iodide by refluxing in an aprotic solvent under an inert atmosphere;
f) condensation of the resulting 2,3-dichlorobenzoyl cyanide of the formula VIII: 
with aminoguanidine bicarbonate in an organic solvent in acidic conditions in the presence of a catalyst at 90xc2x0-125xc2x0 C. followed by insitu cyclisation of the resulting Schiff""s base of the formula IV: 
by refluxing in an aliphatic alkanol in the presence of a base; and
g) purification of the resulting crude lamotrigine of the formula I: 
by a known method such as recrystallisation from an aliphatic alkanol or chromatographic separation
The reduction of 2,3-dichloronitobenzene may be carried out by dissolution of 2,3-dichloronitrobenzene preferably in methanol. The pressure of the hydrogen gas for reduction may be preferably 50-70 psi, still preferably 80 psi and the temperature for the reduction may be preferably 30xc2x0 C. The metal catalysts used in the reduction reaction may be nickel, Raney nickel platinum oxide, rhodium-platinum oxide, palladium-carbon, or palladium salts, preferably Raney nickel. An alkali or alkaline earth metal hydroxide such as NaOH, KOH, Ca(OH)2 or Mg(OH)2 may be optionally used in the reduction reaction.
For the diazotisation of 2,3-dichloroaniline, mineral acids such as HCl or H2SO4, preferably H2SO4, may be used. The diazotisation may be carried out preferably at 0xc2x0 C. The excess sodium nitrite may be optionally decomposed using agents such as urea, sulfamic acid or a small amount of a primary amine dissolved in acid.
The cyano-de-diazonation reaction may be carried out using metal cyanides such as NaCN, KCN or Cu(one)CN or a mixture thereof. Preferably a mixture of Cu(one)CN and NaCN may be used. The cyano-de-diazonation may be carried out preferably at 65xc2x0 C. Excess of cyanide may be optionally decomposed using sodium hypochlorite solution. A phase transfer catalyst such as crown ether or a quaternary ammonium salt in the presence of a nickel catalyst may be optionally used during the cyano-de-diazonation reaction.
The alkaline hydrolysis of 2,3-dichlorobenzonitrile may be carried out using NaOH or KOH in the presence of an aliphatic alkanol such as methanol or ethanol. Preferably methanolic NaOH at reflux temperatures may be used. The unreacted cyano compound may be extracted using toluene, ethyl acetate or a mixture of toluene and ethyl acetate, preferably toluene. Mineral acids such as H2SO4 or HCl may be used for acidic hydrolysis.
2,3-dichlorobenzoic acid may be chlorinated using SOCl2 PCl3 or PCl5. Preferably SOCl2 at 80xc2x0 C. is used.
The cyano-de-halogenation reaction of 2,3-dichlorobenzoyl chloride is carried out under an inert atmosphere such as nitrogen atmosphere. The metal cyanide used may be Cu(one)CN, NaCN, KCN or a mixture of Cu(one)CN and NaCN. The alkali metal iodide may be NaI or KI. Preferably Cu(one)CN in the presence of KI may be used. The aprotic solvent for the reaction may be monochlorobenzene, xylene or any other aprotic solvent, preferably monochlorobenzene.
The condensation of 2,3-dichlorobenzoyl cyanide with aminoguanidine bicarbonate is carried out in the presence of a catalyst such as p-toluenesulfonic acid or a lewis acid catalyst such as AlCl3, TiCl4, FeCl3, ZnCl2, ZrCl4 or any protonated acid such as HCl or H2SO4, in an organic solvent such as toluene or ethyl benzene, in acidic medium using HCl, HNO3 or H2SO4. Preferably toluene and H2SO4 with p-toluenesulfonic acid at 100-120xc2x0 C. may be used. Insitu cyclisation of the Schiff""s base may be carried out in an aliphatic alkanol such as methanol with a strong base such as NaOH, KOH or NaOMe. Preferably methanol and NaOMe may be used.
For the recrystallisation of the crude lamotrigine, an aliphatic alkanol such as isopropanol ethanol or methanol, preferably methanol may be used.
Pharmaceutically acceptable acid addition salts of lamotrigine of the formula I may be prepared by treating lamotrigine of the formula I with acids such as hydrochloric, sulphuric, citric, tartaric, phosphoric, lactic, pyruvic, acetic, succinic, oxalic, fumaric, maleic, oxaloacetic, methane sulphonic, p-toluenesulphonic or benzenesulphonic acid.
According to the invention a new route is employed in the preparation of lamotrigine of the formula I. The substrate for the preparation thereof viz 2,3-dichloronitrobenzene and also the other reagents of the process of the invention are safe, inexpensive and easily available, thus eliminating the use of hazardous and expensive reagent reported in the prior art. The reactions leading to 2,3-dichlorobenzoic acid need not be carried out in a dry environment. Also chlorination of 2,3-dichloro-benzoic acid is conveniently carried out in a non-inert atmosphere without affecting the efficiency of the process. The use of catalyst during reduction of 2,3-dichloronitrobenzene at room temperature proceeds without dehalogenation thereby giving increased yield and purity of 2,3-dichloroaniline. Also the other intermediates of the process of the invention are obtained in good yields and purity. The conversion of 2,3-dichlorobenzoyl chloride to 2,3-dichlorobenzoyl cyanide requires about 6 hours, as against 96 hours reported in a process of the prior art. Similarly the preparation of the Schiff""s base from 2,3-dichlorobenzoyl chloride and further instiu cyclisation of the Schiff""s base to lamotrigine also is less time consuming (8 hrs), as a 7.5-10 days reported in the prior art processes to prepare the Schiff""s base itself. Therefore, the process of the invention is less time consuming and economical. The process of the invention gives a yield of 23% of lamotrigine (starting from 2,3-dichloronitrobenzene) as against a meagre yield of 10% (from 2,3-dichloroiodobenzene) reported in the prior art. Lamotrigine by our invention is also obtained with an excellent purity of 99.67%(by HPLC) after recrystallisation. The process of the invention is, therefore, efficient and economical and also suitable for industrial scale manufacture.